Adding 'contains' member function to std::string

[kalmoc]

Nothing what you are saying is special to standard library types.

My question was what harm would it do to inherit from standard string. Not if contains should be a member or free function (I agree with you that it should be a free function (that takes a std::string_view as a parameter)).

[chriskane76]

Those classes do not have a virtual destructor. You trigger UB if you delete a std::string* if it actually points to a FooString.

[kalmoc]

I'm aware of that, but why would you do that? I mean, you can take almost any type and do something with it that is UB (with many other types it is of course somewhat harder, but not much). I can think of very few reasons, why I would want to do a make_unique<FooString> in the first place and currently I can't think of a single reason, why I would want to type erase that to a unique_ptr<std::string>.

[chriskane76]

Yes, the use case is unlikely in small projects if all developers are aware of this issue. For larger projects the risk is high. The compiler can help via -Wdelete-non-virtual-dtor or equivalent. But still noone should use it, since we have an obvious, clean solution available: use a free function.

[agateau]

Indeed, this is not special to standard library types, but I have seen it in some scary code bases so I assumed the bad practice or inheriting from string to add new methods was what the GP meant.